Digestate-Derived Biochar Modulates Valuable Lactic Acid Production from Food Waste Fermentation: Comparison of Hydrochar and Pyrochar

Abstract Bioconversion of food waste (FW) into lactic acid represents a sustainable strategy for circular resource recovery. However, lactic acid production is often limited by rate-limiting hydrolysis and competitive metabolic diversion in mixed-culture systems. This study systematically investigated the regulatory mechanisms of digestate-derived hydrochar and pyrochar in lactic acid production. Hydrochar and pyrochar significantly enhanced lactic acid production to 17.86 ± 2.76 (HC-20) and 19.06 ± 2.54 g COD/L (PC-15), which increased by 80% and 93% than the blank, respectively. However, the different performance mechanisms were shown in different biochar amendment reactors. Hydrochar primarily enhanced substrate solubilization and diminished volatile fatty acid (VFA) production by acting as a physicochemical buffer. Analysis showed that hydrochar selectively enriched acid-tolerant Lactobacillus. Conversely, pyrochar accelerated substrate utilization through interspecies cooperation, establishing a syntrophic partnership between hydrolytic bacteria and lactic acid bacteria. Additionally, both hydrochar and pyrochar maintained a favorable pH (5.11 ± 0.6) and reductive conditions (ORP < –200 mV). The genes encoding key enzymes (e.g., glucokinase and fructokinase) involved in the Embden–Meyerhof–Parnas (EMP) pathway were 4.05–68.76-fold higher than those of the blank. These findings elucidate the distinct regulatory mechanisms of biochar on mixed-culture microbiomes. Furthermore, the biochar was prepared from fermented residues, highlighting the potential of this approach to achieve resource cycling.